1. Field of the Invention
The present invention relates to a diaphragm of an electroacoustic transducer and a method of manufacturing the same, and more particularly, to materials for a diaphragm of high characteristic speaker with highly balanced internal loss, density and rigidity and a method of manufacturing such diaphragm.
2. Background Art
Various materials have been developed as materials for a diaphragm of an electrodynamic type speaker, for example, for use in conventional audio equipments. Among such materials, typical properties of matter of a diaphragm made of wood pulp, for example, include a density of 0.485, Young's Modulus of 1.17.times.10.sup.10 dyn/cm.sup.2 and internal loss of 0.0714. In general, essential requirements of a material for a diaphragm of such speaker are a small density, high rigidity and large internal loss.
The outline of a relationship between properties of matter of a material for a diaphragm and characteristics of the diaphragm is as described in the following. First, as a density is reduced, a reproducing sound pressure level of the speaker is increased. With an increase in rigidity, a reproducing band of the speaker is expanded, particularly to a high frequency side. In addition, the larger the internal loss becomes, the lower high frequency peak in a reproducing band of the speaker can be brought down to.
To properly meet the above-described three requirements of properties of matter, such diaphragm materials have been conventionally developed as including substances of high rigidity such as carbon fiber and aramid fiber and as having large internal loss such as propylene.
Increasing rigidity of a diaphragm results in a decrease in internal loss and increase in a density. In addition, increasing internal loss tends to cause a reduction in rigidity and an increase in a density.
Highly balancing the above-described three properties of matter to each other is therefore important to make cone paper to be made into a diaphragm of a speaker.
Among the conventional materials of a diaphragm, aluminum has a density of 0.7, Young's Modulus of 62.times.10.sup.10 dyn/cm.sup.2 and internal loss of 0.002, while polypropylene has a density of 0.91, Young's Modulus of 1.08.times.10.sup.10 dyn/cm.sup.2 and internal loss of 0.07.
A diaphragm made of wood pulp having adequately large internal loss and a small density, has an advantage of a narrow frequency band due to lack of rigidity. Producing wood pulp requires deforesting. Restoring forest after deforesting needs great cost and a long period of time.
In a case of a diaphragm made of wood pulp, making a high quality diaphragm out of wood pulp including entangled fibers necessitates a process of beating wood pulp immersed in water for a long time. In other words, beating the wood pulp in water disentangles fiber bundles into fibers of moderate length, which fibers are swelled and torn lengthwise to be easily entangled. Since making of wood pulp requires a process of beating the pulp in water as described in the foregoing, it takes much labor to manufacture such pulp.